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Herbig Ae/Be Stars in nearby OB Associations
We have carried out a study of the early-type stars in nearby OBassociations spanning an age range of ~3-16 Myr, with the aim ofdetermining the fraction of stars that belong to the Herbig Ae/Be class.We studied the B, A, and F stars in the nearby (<=500 pc) OBassociations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1,with membership determined from Hipparcos data. We also included in ourstudy the early-type stars in the Trumpler 37 cluster, part of the CepOB2 association. We obtained spectra for 440 Hipparcos stars in theseassociations, from which we determined accurate spectral types, visualextinctions, effective temperatures, luminosities and masses, usingHipparcos photometry. Using colors corrected for reddening, we find thatthe Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearlydifferent regions in the JHK diagram. Thus, we use the location on theJHK diagram, as well as the presence of emission lines and of strong 12μm flux relative to the visual, to identify the Herbig Ae/Be stars inthe associations. We find that the Herbig Ae/Be stars constitute a smallfraction of the early-type stellar population even in the youngerassociations. Comparing the data from associations with different agesand assuming that the near-infrared excess in the Herbig Ae/Be starsarises from optically thick dusty inner disks, we determined theevolution of the inner disk frequency with age. We find that the innerdisk frequency in the age range 3-10 Myr in intermediate-mass stars islower than that in the low-mass stars (<1 Msolar) inparticular, it is a factor of ~10 lower at ~3 Myr. This indicates thatthe timescales for disk evolution are much shorter in theintermediate-mass stars, which could be a consequence of more efficientmechanisms of inner disk dispersal (viscous evolution, dust growth, andsettling toward the midplane).
| A Comparison of Photometric and Spectral Classification of Some Stars in Perseus
Photometric classification of 18 stars in the areas around thereflection nebula NGC 1333 and the open cluster IC 348 in Perseus, basedon the Vilnius seven-color photometric system, is compared with theclassification based on stellar spectra. In general, a goodcorrespondence of spectral classes is obtained. In some cases, theclassification differences may be the result of the unresolved binarityof stars. Three stars are confirmed to be emission-linepre-main-sequence objects. Interstellar reddening law in the directionof IC 348 is estimated.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| IC 348 proper motion study from digitised Schmidt plates
A new proper motion study for about 1400 stars with R < 18 mag in aone square degree region around the young open cluster IC 348 based onSchmidt plates is presented. With an overall accuracy of about 3 mas/yr(2.5 mas/yr, respectively for all stars with R < 17 mag) we haveobtained membership probabilities dividing our sample into three groups:foreground stars, cluster stars and background stars. This separation isalso supported by the proper motion distribution with respect to thedirection of the standard antapex and an increase of mean proper motioncluster membership probabilities with smaller cluster radii. 240 starsof our sample have cluster membership probabilities larger than 80%. Theresults are compared with the highly accurate proper motion study of\cite[Fredrick (1956]{fredrick56}) and with other catalogues (PPM, ACT,Hipparcos), all containing only bright stars. From PPM and ACT propermotions a cluster radius of about 30 arcmin can be assumed. Hipparcosproper motions and parallaxes allow the separation of foreground stars,cluster stars and background stars in a somewhat larger region around IC348 (with distances from the cluster centre of up to 85 arcmin). On thebasis of Hipparcos data we calculate a mean distance of 261_{-23}(+27)pc for the cluster stars with common proper motion. This is nearly thesame distance as obtained by \cite[de Zeeuw et al. (1999]{dezeeuw99})for the Per OB 2 association covering more than 15 x 10 square degrees.The mean proper motion of the cluster IC 348 obtained in our study is ingood agreement with that of the highly probable members of the Per OB 2association according to \cite[de Zeeuw et al. (1999]{dezeeuw99}).Therefore, we conclude that the cluster IC 348 is embedded in the Per OB2 association. From our proper motion membership probabilities we founda cluster radius of 10-15 arcmin. There is a concentration of Fredrick'shighly probable cluster stars just in between our cluster stars and thedistant field stars proper motion distributions. On the other hand, twogroups of stars in the proper motion diagram of \cite[Fredrick(1956]{fredrick56}) also seem to be indicated if all his possiblecluster members are considered. The X-ray sources from \cite[Preibischet al. (1996]{preibisch96}) identified with optical counterparts fromour proper motion sample show a strong concentration in the propermotion diagram. For these objects which are likely T Tauri stars weobtained high membership probabilities so that we conclude that theybelong to the cluster IC 348 and to the Per OB 2 association. The propermotion foreground stars and cluster members which are counterparts ofnear-infrared (NIR) sources \cite[(Lada & Lada 1995]{lada95}) arelocated in different parts of the (J-H)-(H-K) two colour diagram,respectively.
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| Classification of Population II Stars in the Vilnius Photometric System. II. Results
The results of photometric classification of 848 true and suspectedPopulation II stars, some of which were found to belong to Population I,are presented. The stars were classified using a new calibrationdescribed in Paper I (Bartkevicius & Lazauskaite 1996). We combinethese results with our results from Paper I and discuss in greaterdetail the following groups of stars: UU Herculis-type stars and otherhigh-galactic-latitude supergiants, field red horizontal-branch stars,metal-deficient visual binaries, metal-deficient subgiants, stars fromthe Catalogue of Metal-deficient F--M Stars Classified Photometrically(MDPH; Bartkevicius 1993) and stars from one of the HIPPARCOS programs(Bartkevicius 1994a). It is confirmed that high galactic latitudesupergiants from the Bartaya (1979) catalog are giants or even dwarfs.Some stars, identified by Rose (1985) and Tautvaisiene (1996a) as fieldRHB stars, appear to be ordinary giants according to our classification.Some of the visual binaries studied can be considered as physical pairs.Quite a large fraction of stars from the MDPH catalog are found to havesolar metallicity. A number of new possible UU Herculis-type stars, RHBstars and metal-deficient subgiants are identified.
| New massive X-ray binary candidates from the ROSAT Galactic Plane Survey. I. Results from a cross-correlation with OB star catalogues.
We report the discovery of several new OB/X-ray accreting binarycandidates. These massive systems were found by cross-correlating inposition SIMBAD OB star catalogues with the part of the ROSAT all-skysurvey located at low galactic latitudes (|b|<=20°) and selectingthe early type stars which apparently displayed the most significantexcess of X-ray emission over the `normal' stellar level. The presentsearch is restricted to stars earlier than B6 and X-ray luminosities>=10^31^erg/cm^2^/s. Follow-up optical and X-ray observations allowedto remove misidentified OB stars and spurious matches with interloperX-ray emitters (mostly active coronae) leaving five very likely newmassive X-ray binaries: the O7 star LS 5039 and the Be stars BSD 24-491, LS 992, LS 1698 and LS I +61 235. This latter source was alreadymentioned in an earlier paper. LS 1698 is the probable opticalcounterpart of the hard X-ray transient 4U 1036-56. These new candidateshave 0.1-2.4keV un-absorbed luminosities >=2x10^33^erg/cm^2^/sindicating an accreting neutron star or black hole. On the average theirsoft X-ray luminosities are comparable to those observed from hard X-raytransients in quiescence or from persistent low luminosity Be/X-raysources. The four Be stars have Balmer emission slightly less intensethan previously known systems showing strong outbursts. This suggeststhat the relative weakness of the circumstellar envelope may explain thelow luminosities to some extent. Two additional X-ray binary candidates,HD 161103 and SAO 49725 require further confirmation of their X-rayexcess. Their lower soft X-ray luminosities (1-5x10^32^erg/cm^2^/s)could qualify them as Be + accreting white dwarf systems. Four other Bstars in the Orion and Canis Major OB associations, HD 38087, HD 38023,HD 36262 and HD 53339 exhibit X-ray flux excesses in the range2-7x10^31^erg/cm^2^/s whose origin is unclear. Finally very soft X-rayemission was detected from HR 2875 suggesting the presence of anon-accreting white dwarf companion to the B5 star.
| The Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars
The Henry Draper Extension Charts (HDEC), published in the form offinding charts, provide spectral classification for some 87000 starsmostly between 10th and 11th magnitude. This data, being highlyvaluable, as yet was practically unusable for modern computer-basedastronomy. An earlier pilot project (Roeser et al. 1991) demonstrated apossibility to convert this into a star catalogue, using measurements ofcartesian coordinates of stars on the charts and positions of theAstrographic Catalogue (AC) for subsequent identification. We presenthere a final HDEC catalogue comprising accurate positions, propermotions, magnitudes and spectral classes for 86933 stars of the HenryDraper Extension Charts.
| Interstellar extinction in the direction of the open cluster IC 348 and the Per OB2 association
The relationship between interstellar extinction and distance in thedirection of dark clouds in the areas around the open cluster IC 348 andthe association Per OB2 is determined using the results of photoelectricphotometry of 189 stars in the Vilnius photometric system. Two absorbinglayers are found. The nearest layer, covering the whole area around IC348, shows the mean extinction A(V) of about 0.7 mag. It begins at thedistance of 160 pc and probably is an extension of the Taurus darkclouds to the northwest. The second absorbing layer has the form of achain of dark condensations named L1468, L1470, and L1471 and is at 260pc distance. This layer has a higher density, its mean extinction beingabout 2.0 mag. The cluster IC 348 is at about the same distance and isphysically related to the dark cloud L1470. The distance of the Per OB2association is found to be 340 pc and the mean extinction of its membersis 0.95 mag. A model of the spatial distribution of the Perseus andTaurus dark clouds based on photometric distance determinations in thisand previous papers is proposed. Six stars in the IC 348 area aresuspected to have emission in the H-alpha line.
| Peculiar stars in the association Per OB2.
Not Available
| Radial Velocities of B-Type Stars in the Nearest Associations.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1963ApJ...137..791B&db_key=AST
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| Takýmyýldýz: | Kahraman |
| Sað Açýklýk: | 03h45m39.16s |
| Yükselim: | +32°26'24.2" |
| Görünürdeki Parlaklýk: | 9.15 |
| özdevim Sað Açýklýk: | 6.6 |
| özdevim Yükselim: | -4 |
| B-T magnitude: | 9.994 |
| V-T magnitude: | 9.22 |
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